Nitrogen reserves and flows in an inland northwest dry-site forest.

1. Nitrogen reserves and flows in an inland northwest dry-site forest. George Scherer, Jeff Hatten, Darlene Zabowski , University of Washington Introduction Land managers in northwest inland areas are restoring forested areas to improve growth and sustainability by using prescribed fire and thinning. Nearly 100 years of fire exclusion have resulted in multiple canopy structures of mixed woody species and spurred controversy regarding wildfire risk and emerging forest health issues. We hypothesized that such a long interval since fire disturbance has created biomass and nutrient accumulations with minimal losses and contributes to sustainability. Also, information on nutrient cycling for dry inland forests is far more limited than for humid coastal areas. We studied N since it is relatively easy to measure and it is typically the most limiting nutrient for forest productivity in this region. Methods Belowground, we described soil pits and removed core and bulk samples by horizon to ~ 90cm for chemical analysis, bulk density, and volume determinations. Aboveground, total C and N were determined for trees, grass, herbs, shrubs, and forest floor in 0.1 ha plots. Biomass was determined by allometric equations from weighed and dried core samples from trees and 0.1-m2 harvest transects for understory components. Available NH4- N, NO3-N was determined with resin core lysimeters. Deposition and through-fall was made with funnel bottle collectors and N loss to stream was determined by stream flow estimate. N ions were KCl extracted and determined on AutoAnalyzer with NH4 reported here. C and N totals were determined with Perkin Elmer CHN analyzer. Coarse wood debris (CWD) volumes were measured for size greater than ~7cm diameter. Results and Discussioncont. Tree components dominate the 149 Mg ha aboveground biomass of the Mission Cr forest. For year 2003, the above ground N pool was 547 kg ha. Mission aboveground N pool is about 30% greater than other inland forests, assuming a similar disturbance history (Table 2). Understory vegetation is presently a minor contributor to biomass and N pool. CWD was 2 % of total biomass here. The CWD N pool was 3 % of aboveground total. For this age of forest we expected to find greater quantities of CWD. Similar aged forests in northwest have reported CWD as much as 25-30 % of the biomass. Mission Cr. soil-forest floor N pool was 5700 kg ha. The forest floor and soil N pool is about the same as the Wyoming lodgepole and Arizona ponderosa interior forests. An AB horizon held the greatest amount of N at this site. It was also the thickest horizon in most profiles and typically contained common to many roots. C: N ratios decline with depth of soil profile. AB horizon, however, has a high OM value and a high N content of 2630 kg ha. this suggest a zone in soil of potentially optimum nutrient availability for plant use. AB was usually found greater than 10 cm in depth below the reach of heat from fire. Figure 3. Biomass, N pools and fluxes in Mission Cr Pipo/ Psme ca 2003. Biomass N Mg ha kg ha Lv 9 99 Br 28 168 St 60 79 Rt 48 185 Veg 1 1 CWD 3 15 Tot: 149 547 kg N ha yr Bulk Ppt. 2.23 Thrufall 0.26 Litterfall 11.4 Stream 1.1 Figure 2. Current stand condition, study area. Site Study sites are mixed forests of Grand-fir, Douglas-fir, and ponderosa pine located near Wenatchee in the Mission Creek watershed on ~40 million yr old Swauk sandstone. Slopes range from 15 to 70% with mostly southerly aspects. Soils orders are fine textured Mollisol, Inceptisol, and Alfisols. Figures 1 and 2 show location and current stand condition. Lightning ignited wild fire is the major landscape disturbance. Annual precipitation is < 60 cm falling mostly as snow from Nov to Feb. Average age of trees on study units ranged 65 to 80 years. Basal area of study plots was 46-59 m2 / ha. Avail N kg N ha yr O 2.6 A 0.76 B 0.76 • Conclusions • Biomass and N accumulation for this site appears to be similar to that of other northwest forest systems. Belowground organic matter pool is roughly similar to aboveground biomass but soil and forest floor N is 10 times that found above ground. This implies a belowground N conservation mechanism for this predominantly fire disturbed forest system. • Substantial belowground biomass and N pools are found in AB horizons lower than 10 cm the apparent rooting zone. • Bulk precipitation inputs of N seem elevated in this forest. This may be from seasonal pulses of thunderstorm activity due to our sample collection method. More work needed here. • Litterfall biomass inputs are the largest source of forest floor and soil N. Available NH4-N results suggests that substantial amount of the litter N remains unavailable in the short term. N may be conserved, but mechanisms of its release remain unclear. • This system may retain about 12 kg N ha yr if estimates of N flows are true and the forest remains undisturbed by fires. This accumulation could explain, in part, the relative N fertility of this dry site forest. Another factor is fine textured soils with volcanic ash influence. • Additional work will be needed to evaluate the CWD volumes at this site to understand its fate and decomposition process role. • Using information from other forest systems to plan management activities is common, but has limitations of varying site properties, study methods and site objectives. Site information such as this study can measurably improve land manager decision making for restoration activities. However, there still may be controversy over the methods of biomass fuel removal by prescribed fire and thinning on meeting sustainability goals. Soil OM N Mg ha Hor cm kg ha C:N O 31 6 570 31 A 38 11 1060 21 AB 68 41 2680 15 Bt 38 33 1400 15 Tot: 175 5710 George Scherer Table 1. N fluxes in other northwest forest systems (kg N ha yr). George Scherer Figure 1. Study location at X. Note no continental glaciation at study area. Sandstones aged about 40 myn yr bp suggests very old soils. Results and Discussion For the one-year that we measured, we found bulk precipitation input of 2.2 kg N ha yr, thrufall 0.2 kg N ha yr, litterfall 11.4 kg N ha yr, and stream/ soil loss of 1.1 kg N ha yr. These values generally agree with those of other northwest forest systems (Table 1). High bulk ppt at Mission site was likely influenced by inputs of 10-14 kg N ha yr in May to June thunderstorm months. Tree litterfall is the greatest source of N to this forest floor and soil component; total litterfall was 2008 kg ha yr. However, the O horizon NH4-N accumulation rate was 2.6 kg ha yr, indicating less than 20% of litterfall N becoming available N (assuming bulk ppt N is readily available). N rates in lower horizons are about one-third that of the O horizon suggesting less activity or greater utilization by system biological components. Table 2. Biomass and N aboveground and soil for inland and northwest forest systems. X We acknowledge funding support from USDA, USDI Joint Fire Sciences Program. R. Sletten